Minimum thickness of carbon coating for multipacting suppression

We performed a combined secondary electron yield and X-ray photoelectron spectroscopy study on a prototypical system formed by increasing coverages of amorphous carbon (a-C) deposited on atomically clean Cu. A remarkably thin a-C layer, of about 6-8 nm, is surprisingly enough to lower below 1 the secondary emission yield of the whole system. This feature qualifies such low thickness coatings as a optimal multipacting suppressor that will not significantly affect impedance issues. The concomitant reduction of surface conductivity observed after antimultipacting coating is, in fact, a major drawback, reducing its applicability in many research fields. The consequences of this observation are discussed mainly for a-C coating applications to mitigate detrimental multipacting effects in radio-frequency devices and accelerators, but are expected to be of interest for other research fields and to hold for other conductive substrates and overlayers.